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Reviewing a subject is done to provide an insight into theoretical and conceptual background of the study. Looking back into the history of an emerging field and summarizing it in a few pages is a herculean task. Anyway, it was imperative to write a few words about the rise of silicene, its properties, and its applications as gas sensors. Currently, silicene is a growing field of interest. It is probably one of the most studied materials nowadays and scientists and researchers are studying it because of its intriguing electronic properties and potential applications in nanoelectronics. Various experimental and theoretical investigations are going on worldwide to explore the various aspects of this field. It is essential to review the literature based on investigations by various scientists in this field.

Density functional theory-based first-principles investigation is performed on pristine and mono vacancy induced GaAs nanoribbons to detect the presence of three volatile organic compounds (VOCs), aniline, isoprene and o-toluidine, which will aid in sensing lung cancer. The study has shown that pristine nanoribbon senses all three analytes. For the pristine structure, we observe decent adsorbing parameters and the bandgap widens after the adsorption of analytes. However, the introduction of the carrier traps induced by defect causes deep energy wells that vary the electrical properties as indicated in the bandgap analysis of GaAs, wherein adsorption of aniline and o-toluidine reduces the bandgap to 0 eV, making the structure highly conductive in nature. The adsorption energies of defect-induced nanoribbon are more as compared with the pristine counterpart. Nonetheless, the introduction of defects has improved the sensitivity further.

A different idea to find out the position of the nodes which are referred to as target nodes in the area of wireless sensor networks has been presented in this paper. Here only one node is kept mobile which is the anchor node (AN) and with the help of this AN, we will find out the location of all the target nodes (TN) which are kept stationary in the sensing field. The AN will follow a curve on the basis of Hilbert trajectory. Virtual anchor (VA) nodes are projected, which will help in locating the target nodes. The algorithm used to optimize this technique is biogeography based optimization (BBO). The main objective of this paper is to reduce the issue of LOS (line of sight) and this has been achieved with the help of deployment of VA nodes.

Density functional theory-based first-principles investigation is performed on pristine and mono vacancy induced GaAs nanoribbons to detect the presence of three volatile organic compounds (VOCs), aniline, isoprene and o-toluidine, which will aid in sensing lung cancer. The study has shown that pristine nanoribbon senses all three analytes. For the pristine structure, we observe decent adsorbing parameters and the bandgap widens after the adsorption of analytes. However, the introduction of the carrier traps induced by defect causes deep energy wells that vary the electrical properties as indicated in the bandgap analysis of GaAs, wherein adsorption of aniline and o-toluidine reduces the bandgap to 0 eV, making the structure highly conductive in nature. The adsorption energies of defect-induced nanoribbon are more as compared with the pristine counterpart. Nonetheless, the introduction of defects has improved the sensitivity further.

Using non-equilibrium Green’s function (NEGF) formalism combined with first-principle density functional theory (DFT), we explore the nature of adsorption of carbon-based gases and the resulting structural, electronic (band structure, density of states, Mulliken population, and electron density), and transport properties (current-voltage characteristics and transmission eigenstates) on pure and defected armchair silicene nanoribbons (ASiNRs) for sensing applications. It is observed that CH4 and CO2 are weakly adsorbed on pristine (P-ASiNR) as well as defective (D-ASiNR) nanoribbons owing to their low adsorption energy and charge transfer, thereby exhibiting low sensitivity and high recoverability. On the other hand, CO is chemisorbed on both nanoribbons exhibiting greater adsorption energy and current, thereby having more sensitivity and more recovery time. Mulliken population analysis reports that a significant amount of charge transfer prevails between ASiNR and gas molecules, validating our results for adsorption energies of the systems. CO2 and CO donates charge to the ASiNR, showcasing their electron-donating nature; contrariwise, CH4 behaves as electron-withdrawing gas by accepting electronic charge from ASiNRs. Our calculations reveal that introduction of vacancy defect increases the sensitivity significantly which is promising for future gas-sensing applications as well.

In this paper, the electronic and transport properties of armchair silicene nanoribbons (ASiNRs) are analyzed for their application as highly selective and sensitive gas molecule sensors. The study is focused on sensing three nitrogen based gases; ammonia (NH3), nitrogen dioxide (NO2) and nitric oxide (NO), which depending upon their adsorption energy and charge transfer, form bonds of varying strength with ASiNRs. The negligible band gap of ASiNRs is tuned by adding a defect in ASiNRs. Adsorption of NH3 leads to the opening of band gap whereas on adsorption of NO2 and NO, ASiNRs exhibit metallic nature. Distinctly divergent electronic and transport properties of ASiNRs are observed and on adsorption of NH3, NO2 and NO, renders them suitable for sensing them. All gas molecules show stronger adsorption on defective ASiNRs (D-ASiNRs) as compared to pristine ASiNRs (P-ASiNRs). The work reveals that introduction of defect can drastically improve the sensitivity of ASiNRs.

Using density functional theory (DFT) and non-equilibrium Green’s function (NEGF) formalism, the electronic and transport properties of ammonia (NH3) molecule adsorbed on armchair silicene nanoribbons (ASiNRs) are calculated. Different variants of ASiNR have been considered viz. pristine, defective, Al-doped, and P-doped. It has been observed that though the pristine ASiNR is not much sensitive to this gas molecule, but its sensitivity can be drastically enhanced by introducing defects and dopants. NH3 gas molecule exhibits stronger adsorption on ASiNRs with addition of defect and dopants. The findings are suggestive of defective and Al-doped ASiNRs being more suitable as sensors for NH3 owing to the strong adsorption and large charge transfer of the gas molecule with these ASiNR variants whereas NH3 exhibits physisorption on pristine and P-doped ASiNRs possessing minimal adsorption energy and charge transfer as well. Defective ASiNRs are found to exhibit the strongest adsorption of all resulting in higher current as well. The study indicates that modified ASiNRs are potential candidates for nanoelectronic NH3 gas sensors.

A study is done to check the sensing functionality of armchair zinc oxide (ZnO) nanoribbon towards uric acid. The main focus of the research is to observe the change in the electronic properties (adsorption energy, bandstructure and density of states) and transport properties (current-voltage characteristics) of nanoribbon on adsorption of uric acid. In this work, two armchair ZnO nanoribbons of width, N = 4 and 6 atoms are used, and additional variations are created in the nanoribbon by introducing defect and doping agent. Manganese is used as a dopant. The work reveals that chemisorption occurs only in the case of doping for both widths of nanoribbons, and there is an enormous increase in the conductivity of defective armchair ZnO nanoribbon with width, N = 6 as compared to others on adsorption of uric acid. All calculations are carried out using density functional theory (DFT) and non-equilibrium Green's function (NEGF). Graphical abstract.

Adsorption of toxic hydrogen cyanide gas (HCN) on armchair silicene nanoribbons (ASiNRs) is investigated by the first principles method using density functional theory (DFT) to compute geometric, electronic, and transport properties. Two variants of ASiNRs are considered: pristine ASiNR (P-ASiNR) and defective ASiNR (D-ASiNR), which is created by introducing a vacancy in P-ASiNR by removal of a Si atom. Total energy optimizations are used to find the most stable structures. The calculated results reveal that although HCN is physisorbed in both variants, sensitivity in the case of D-ASiNR is sufficiently enhanced owing to more adsorption energy and charge transfer between the ASiNR-gas complex. Also, the inspection of current-voltage characteristics demonstrates that the introduction of defect has considerably increased the conductivity of ASiNR. Hence, D-ASiNR may be used as a promising sensor for HCN gas. Graphical abstract Transmission eigenstates of (a) Pristine ASiNR (b) Defective ASiNR after HCN adsorption.

The adsorption behavior of sulfur-based toxic gases (H2S and SO2) on armchair silicene nanoribbons (ASiNRs) was investigated using first-principles density functional theory (DFT). Being a zero band gap material, application of bulk silicene is limited in nanoelectronics, despite its high carrier mobility. By restricting its dimensions into one dimension, construction of nanoribbons, and by introduction of a defect, its band gap can be tuned. Pristine armchair silicene nanoribbons (P-ASiNRs) have a very low sensitivity to gas molecules. Therefore, a defect was introduced by removal of one Si atom, leading to increased sensitivity. To deeply understand the impact of the aforementioned gases on silicene nanoribbons, electronic band structures, density of states, charge transfers, adsorption energies, electron densities, current-voltage characteristics and most stable adsorption configurations were calculated. H2S is dissociated completely into HS and H species when adsorbed onto defective armchair silicene nanoribbons (D-ASiNRs). Thus, D-ASiNR is a likely catalyst for dissociation of the H2S gas molecule. Conversely, upon SO2 adsorption, P-ASiNR acts as a suitable sensor, whereas D-ASiNR provides enhanced sensitivity compared with P-ASiNR. On the basis of these results, D-ASiNR can be expected to be a disposable sensor for SO2 detection as well as a catalyst for H2S reduction. Graphical abstract Comparison of I-V characteristics of pristine and defective armchair silicene nanoribbons with H2S and SO2 adsorbed on them.

Due to the growth of nanotechnology, many miniature devices have been developed which are capable of performing only limited functionalities. A nanomachine is considered to be the most basic functional unit at nanoscale. Nanomachines are very minute components which consist of an arranged set of molecules that are capable of performing very easy tasks. Nanomachines can be connected together to form a nanonetwork, which expands their capabilities so that they may perform more complicated tasks. Nanonetworks play an important role in applications such as industry, military, biomedical, environmental monitoring etc. The four main techniques on nanoscale communication are: nanomechanical, acoustic, molecular or chemical and electromagnetic communications. However, this paper focuses on the molecular communication as the promising nanoscale communication approach.